In imaging using an ultrasonic wave, image data and time-series data are generated based on the reflected wave (echo) of an ultrasonic wave emitted to an object. Since ultrasonic waves are partially reflected on a boundary where the acoustic impedance is different while propagating through the object, echo having amplitude depending on the impedance difference is generated. Accordingly, the boundary surface can be displayed as a tomographic image of the object. In this case, waveform distortion occurs due to the characteristics that the ultrasonic wave propagates faster in a portion with high sound pressure and propagates slowly in a portion with low sound pressure.
Due to this waveform distortion, a nonlinear component formed of a harmonic or a difference frequency is generated in addition to the fundamental wave component of the emitted acoustic wave. Compared with a normal imaging method in which all echoes including the fundamental wave component are used for imaging, when only the nonlinear component is used for imaging, it is possible to more emphasize the difference between light and shade in the image. Therefore, it is possible to obtain a high-resolution image. Such imaging for imaging the nonlinear component of the body tissue is called tissue harmonic imaging (THI).
In the THI, as a method of extracting the nonlinear component from the echo, there is a method called an amplitude modulation method (for example, refer to PTL 1). In general, the fundamental wave component is proportional to the amplitude, and the nonlinear component generated as a waveform distortion component is proportional to the square of the amplitude of the transmitted fundamental wave sound pressure. The amplitude modulation method is a method of canceling the fundamental wave component using this. Specifically, the sound pressure level (amplitude) of the second transmission is set to 1/k (k is an integer of 1 or more) of the first transmission, and the echo is multiplied by k and the result is subtracted from the echo of the first transmission, thereby obtaining a reception signal (reception beam) excluding the fundamental wave component.